IR imaging systems that are responsive to two or more spectral bands presently require separate and additional detectors, electronics, spectral filters, and other optical components for each spectral band. The resulting size, weight, power, and complexity hinders the development and/or production of compact and/or affordable multi-spectral (color) IR imaging systems. Such systems are typically required for use in numerous critical applications, such as missile interceptors, missile launch detection/warning, IR search and track, automatic target recognition, and all climate situational awareness. However, many of these applications require very close registration/alignment between the spectral bands within the composite multi-spectral image (both spatial and temporal), and conventional techniques are typically not well suited for serving such critical and demanding applications.
It is known to attempt to solve these problems with multi-spectral starting (as opposed to scanned) IRFPAs having several design forms. These design forms have in common an array of pixels, typically photodiodes, that detect radiation in two distinct spectral bands. The resulting signals from each band are then read out separately. These designs differ in details, but are generally divided into two categories, each having particular limitations in meeting the system requirements outlined above.
So-called “sequential” designs detect/integrate and read out IR radiation in one band during one frame time (e.g., 16.67 milliseconds), and then detect/integrate and read out the other band during the next frame time. As such, the temporal mis-registration (or delay) between the detection of the two spectral bands is equal to the frame time of the sensor system.
More specifically, previous IRFPA designs have operated the detector in the sequential mode by switching the detector bias on alternate (sequential) frame periods. At least one type of conventional sequential two color ROIC unit cell contains a single capacitor, connected to the detector through a pair of direct-injection (DI) field effect transistors (FETs), such as MOSFETs. As the detector and DI MOSFET biases are switched on alternating frames the sequential ROIC/IRFPA system integrates and reads out one spectral band per frame. This results in the above-mentioned temporal mis-registration or delay between the two bands that is equal to the frame time, typically 16.67 ms. This conventional technique may be analogized to the use of a conventional rotating filter-wheel approach.
In contrast, so-called “simultaneous” designs operate so as to detect/integrate in both spectral bands simultaneously, but they require two separate contacts between the two IR detectors and their corresponding ROIC components for each pixel (i.e., in each ROIC unit cell). In this case the required pixel size and/or cost is increased over that of the sequential two color IRFPA approach, and over that of the conventional single color IRFPA for that matter. The second detector contact per pixel, which requires additional unit cell area, typically prevents an optimum spatial registration (or co-location) between the two sensed spectral bands.
Reference can be had to the following U.S. Patents for teaching various aspects of multi-spectral IR detectors: U.S. Pat. No. 5,113,076, May 12, 1992, “Two Terminal Multi-Band Infrared Radiation Detector”, by Eric F. Schulte; U.S. Pat. No. 5,373,182, Dec. 13, 1994, “Integrated IR and Visible Detector”, by Paul R. Norton; and U.S. Pat. No. 5,731,621, Mar. 24, 1998, “Three Band and Four Band Multispectral Structures having Two Simultaneous Signal Outputs”, by Kenneth Kosai, the disclosures of which are incorporated by reference herein in their entireties.